The present invention relates to a system and method for determining the time interval ("dead time") between adjustment of a parameter in a process and detection of the effect of such adjustment on the product of the process. The present invention also relates to control of a process using a dead time compensated control loop.
In a typical manufacturing process, there may be a number of actuators at a particular processing station which can be utilized to vary process parameters, such as temperature, pressure, flow rate, etc., to thereby effect changes in the process. In addition, in some processes, there are also a number of sensors which are used to detect the effects of varying the process parameters on the resulting product. Some of these processes make use of feed-back signals from the sensors to control the actuators and thereby control the manufacturing process in accordance with particular process specifications.
FIG. 1 shows a schematic arrangement of a simple product manufacturing process. In the process, a particular product 6 is being processed by a processing station 4. The product is carried by a conveyor belt 3 in a direction indicated by arrow 7 to a sensor 2. The sensor 2 determines if the processing station 4 has processed the product 6 in a manner meeting a desired specification, e.g., a target weight or dimension. The sensor 2 transmits feedback signals, corresponding to the sensed characteristic of the product 6, along line 8 to a controller 5. If the controller 5 determines that the product 6 does not meet the desired specification, it outputs a control signal along line 9, to the processing station 4 so as to adjust one or more parameters such that products subsequently processed by the processing station 4 will meet the desired specification.
It can be seen from FIG. 1 that there is a time interval, T, between the change in parameter at the processing station 4 and the detection of such change by the sensor 2. The interval T is present due to the physical characteristics of the system, i.e. in this example, the speed V of the conveyor belt 3 and the distance D between the processing station 4 and the sensor 2 (T=D/V). This interval T represents the time between when the processing station 4 has processed the product 6 and when the sensor 2 can detect the effects of the processing station 4 on the product. This time interval T is often referred to as "dead time". The term "dead" is used instead of "delay" to describe the time interval T because the interval T is neither a delay in the response of the sensor 2 nor a delay in the response time of the processing station 4 or controller 5. Rather, the dead time interval T is a waiting period during which the particular product 6 cannot be sensed by the sensor 2 during the movement of the product 6 from the processing station 4 to the sensor 2.
It has been found that for certain applications it is critical to be able to accurately determine the dead time T. In the process shown in FIG. 1, for example, assume that the controller 5 effects a change in the processing parameters of the processing station 4 at the time that the product 6 is under the station 4. If the controller 5 reacts to signals from the sensor 2 after a time interval less than the actual dead time T since the change in the process parameter, then the process will be over-corrected. This is because the feed-back signal from the sensor 2 would not correspond to the product 6 which was actually processed by the processing station 4 during the change in the process parameter. Instead, the sensor would have detected a different product 6' which is in front of the product 6 on the conveyor belt 3. Product 6' was, however, not affected by the change in the process parameter and therefore does not correspond to the current state of the process at the instant the sensor 2 detects product 6'. As a result, the feed-back signal from the sensor 2 will erroneously indicate to the controller 5 that further corrections of the process parameter are required in order to make the subsequent products meet the desired specification. Consequently, an additional, and possibly unnecessary correction of the process parameter is triggered by the controller 5, which will thus over-correct the process. Such over-correction may result in oscillations in the control process.
On the other hand, if the controller 5 reacts to the feed-back signal from the sensor 2 at a time interval which is longer than the actual dead time T since the preceding change in the process parameter, the controller 5 would thus control the processing station based upon sensor feed-back signals which correspond to the physical characteristics of a product 6'" that was processed by the processing station long after product 6 had been processed. As a result, an unnecessarily large number of products 6" may have passed from the processing station 4 to the sensor without the controller 5 making corrections to the processing characteristics of the processing station 4. In the interim, the process parameter affected by the processing station 4 may have drifted out of the specification limits.
Referring to FIG. 2, a previously known method of determining dead time in a paper manufacturing process will now be described. In particular, FIG. 2 illustrates a system in a papermaking process for controlling the mass per unit area ("basis weight") of a sheet of paper 13. Typically, in a papermaking process, a slurry of wood fibers and water is fed into a tank called a "headbox" 10, and the slurry then flows continuously through an opening 12 defined by the slice lip 16. The slurry is deposited onto a continuous conveyor belt 14. The conveyor belt 14 moves in a direction away from the headbox 10 as shown by arrow 15. The slurry thus forms a continuous sheet 13 on the conveyor belt 14. The sheet of paper slurry 13 drains some of its water content as it is being transported by the conveyor belt 14, and thereafter the sheet 13 is pressed by rollers 20 to remove additional moisture from the sheet 13. The basis weight of the sheet 13 is then measured using the basis weight sensor 24.
The vertical position of the slice lip 16 is related to the size of the opening 12 and hence to the amount of slurry deposited on the conveyor belt and ultimately to the basis weight of the sheet 13. The vertical position of the slice lip 16 is controlled by a plurality of actuators 18 which are connected to the slice lip 16 and to the headbox 10. Information from the sensor 24 is transmitted to a controller 26 which in turn controls the actuators 18 to obtain the desired basis weight of the sheet 13.
A previously known method of determining the dead time with respect to the slice lip 16 and the sensor 24 includes the steps of manually perturbing the control of the actuators 18 and determining the elapsed time from such a single perturbation before the effect of the perturbation on the basis weight can be detected by the sensor 24. Specifically, the perturbation effects a sudden change in width of the opening 12 from its nominal width, thereby changing the basis weight of the sheet 13. The local change in basis weight serves as a marker to be detected by the sensor 24 after dead time T. The magnitude of the perturbation is set to be substantially larger than the magnitude of the noise in the control signals 28 from the controller 26 to ensure that the sensor 24 is able to distinguish the marker, which is caused by the manual perturbation, from the variations in basis weight caused by noise in the control signals 28.
The above described method of determining dead time has a number of shortcomings. Firstly, the method requires a relatively large local change in basis weight from the steady state basis weight to ensure that the marker can be clearly distinguished from the variations in basis weight caused by noise in the control signals. As a result, considerable time may be required after the change for the process to regain a steady state.
Secondly, the above described method destroys the uniformity of the finished product. The presence of the marker sets the upper limit on the length of sheet that can be produced of uniform basis weight. Moreover, when the sheet is cut to size for conversion into finished paper products, the local basis weight variation may be located in the middle of a cut portion. Such portion has to be discarded, thus leading to waste. Also, additional inspection steps are required to identify such marked portion in the finished product.
Another method previously used to determine dead time involves determining the speed of the sheet 13 by simply determining the speed of the rollers 20. However, the dead time determined by this method is not reliable. This is because there may be slippage between the rollers 20 and the paper sheet 13, which slippage may vary with time. In addition, the sheet 13 changes length as it is stretched and/or pressed by the rollers 20. The speed of the rollers 20 thus does not directly correspond to the speed of the sheet 13. Consequently, dead time cannot be accurately determined by this method.